Recently, as systems for efficiently transmitting moving image signals in a narrow band by compression-coding, there are known moving image coding systems such as H.261, H.263 and MPEG (Moving Picture Experts Group)-4. These moving image compression-coding systems are standardized internationally by ITU-T (International Telecommunication Union Telecommunication Standardization Sector) recommendations or ISO (International Organization of Standardization)/IEC (International Electrotechnical Commission).
These moving image compression-coding systems such as H.261, H.263 and MPEG-4 have different characteristics, respectively. Therefore, in a case where a transmission side moving image coding device and a receiving side moving image decoding device adopt different moving image compression-coding systems, when moving image signals are transmitted/received between the transmission side moving image coding device and the receiving side moving image decoding device, there is required a moving image compression-coding system conversion device, that is, a transcoder.
The plural moving image compression-coding systems described above have such similar aspects that inter-frame prediction by motion compensation, DCT (Discrete Cosine Transform) and quantization, and variable-length coding using Huffman code are performed. However, bit streams, compression-coded actually, are largely different in the respective moving image compression-coding systems.
Therefore, a transcoder includes a decoder and an encoder. A transcoder decodes a compression-coded moving image signal, received from a transmission side moving image coding device by the decoder, and then re-encodes the decoded signal by the encoder, and outputs the compression-coded moving image signal to a receiving side moving image decoding device.
The conventional systems having the configuration described above have the following problems.
That is, when a coded bit stream is converted as described above, a transcoder obtains information about a moving image compression-coding system adopted by a receiving side moving image decoding device. In a case where compression-coding is performed by using MPEG-4, for example, as a moving image compressing coding system, the transmission side moving image coding device performs compression-coding by using at least one of plural parameters (coding tools, etc.) included in DCI (Decoder Configuration Information) of MPEG-4. On the other hand, the receiving side moving image decoding device performs decoding by using at least one of plural parameters (coding tools, etc.) included in the DCI of MPEG-4.
Relating to MPEG-4 described above, in order to prevent the quality of decoded moving image from being deteriorated significantly due to a bit error caused when the coded bit stream is transmitted for example, coding tools such as Resync Marker, Data Partitioning, and Reversible VLC (Variable Length Codes) are stipulated in the recommendations mentioned above.
A conventional transcoder described above could not obtain information about parameters for the receiving side moving image decoding device to perform decoding. Therefore, when a conventional transcoder is used, there has been a problem that a coded bit stream converted by the transcoder cannot be decoded and reproduced by the receiving side moving image decoding device due to the difference in the parameters (coding tools, etc.) of moving image compression-coding systems between the transcoder and the receiving side moving image decoding device.
Further, as a characteristic of the moving image compression-coding, intra-frame coding is performed to the first frame of a moving image signal, and inter-frame prediction coding is performed to the subsequent frames generally. If the first frame cannot be decoded and reproduced correctly, the receiving side moving image decoding device cannot decode and reproduce subsequent, inter-frame prediction coded frames correctly. Alternatively, if intra-frame coding is performed periodically for example, the receiving side moving image decoding device cannot decode and reproduce the moving image signal until data of the intra-frame coded frame is inputted.
That is, there has been a problem that when a coded bit stream outputted from the transcoder is inputted in the receiving side moving image decoding device earlier than the time when the receiving side moving image decoding device starts decoding and reproduction of the coded bit stream, the receiving side moving image decoding device cannot decode and reproduce the first, intra-frame coded frame correctly. As described above, there has been a problem that once decoding and reproduction of an intra-frame coded frame are failed, decoding and reproduction of the subsequent, inter-frame prediction coded frames cannot be performed until decoding and reproduction of the next, intra-frame coded frame are performed successfully.
An object of the present invention is to provide a moving image compression-coding system conversion device (transcoder) capable of decoding and reproducing moving image signals in receiving side moving image decoding devices even when parameters used for decoding in the receiving side moving image decoding devices are different in the respective receiving side moving image decoding devices, or even when the parameters are changed in the receiving side moving image decoding devices.
Another object of the present invention is to provide a moving image compression-coding system conversion device (transcoder) capable of outputting a coded bit stream in which the first frame is intra-frame coded and the subsequent frames are inter-frame prediction coded, enabling a receiving side moving image decoding device to decode and reproduce a coded bit stream correctly right after starting decoding and reproduction of the coded bit stream.